The invention relates to a monochrome cathode ray tube comprising at one end an electron gun and at the oppositely-located end a display screen having a phosphor layer. A deflection unit is provided around the neck of the display tube and a magnetic focusing device is provided between the elecron gun and the deflection unit, around the neck of the display tube for generating a static focusing field.
For focusing electrons in a cathode ray tube two types of lenses are used, electrostatic ones or magnetic ones. In order to achieve a high definition it is desired to achieve a very good extent of focusing to produce a small electron spot and thus obtain high resolution. Magnetic lenses may generally be provided outside the neck of the tube, this in contrast with electrostatic lenses which are present in the neck of the tube. As a result of this the diameter of the magnetic lenses may be larger, which provides a better lens quality because spherical aberration decreases when the lens diameter increases. A smaller spherical aberration leads to a smaller spot on the screen, which is required for high resolution guns. For higher resolution e.g. (projection) television tubes a magnetic focusing lens is hence preferably used.
When magnetic focusing lenses are used, they may be distinguished between two types: electromagnetic lenses, and magnetostatic lenses. In an electromagnetic lens a field is generated by a coil enclosed partly by a yoke. In a permanent magnetic lens the field is generated by a permanent magnetic material whether or not provided with a yoke (DE-PS No. 891 119). The electron beam is also moved over the screen by a deflection coil, the intensity of the beam being modulated to obtain a picture. The great beam angular aperture which is associated with the use of a focusing lens having a large diameter has for its result that upon deflecting the beam by the deflection coil the electron spot on the screen is not only enlarged but also distorted. As a result of this deflection defocusing an elliptical spot having a diameter which is larger than that in the centre of the display screen is formed at the edge of the screen.
In some applications of cathode ray tubes, for example projection television tubes or so-called data graphic display tubes, such a distortion cannot be tolerated.